In the past, various techniques have been proposed for selective attenuation of harmonic frequencies created in plasma processing systems. These techniques utilize either a low-pass filter or a trap circuit. For example, U.S. Pat. No. 5,302,882 entitled “LOW-PASS FILTER FOR PLASMA DISCHARGE” discloses such techniques.
In the plasma processing industry, capacitively coupled plasma sources are widely used for dry etching and plasma enhanced chemical deposition. Dry etching is a process for removing a layer of material from a wafer surface. This removal is a result of combined mechanical and chemical effects of high-energy plasma ions striking the substrate surface In plasma enhanced chemical deposition, a layer of a material is deposited on the substrate surface. This material is introduced into the plasma either by sputtering a target made of the material or by supplying a gas which contains the material or from which the material is produced by a chemical reaction. The material may be ionized by the plasma and can then be attracted to the substrate by an electric field.
Plasma processing is commonly used in the semiconductor fabrication industry. The trend in the semiconductor fabrication industry has been toward integrated circuits having smaller elemental features. As a result, etch and deposition rate uniformity over the wafer surface has become more important, particularly when a layer is being etched or deposited according to a pattern. At the same time, recent developments in plasma source technology have led to the increased use of very high frequency RF excitation, e.g., from 60 to 300 MHz, and possibly even higher, to initiate and sustain the plasma.
The use of these very high excitation frequencies provides a benefit in the form of increased power coupling to the plasma, and thus excitation efficiency, that is likely caused by an increase of plasma electron temperature. This increase of RF power coupling affects the plasma density and the harmonic generation in the plasma. However, maintaining high etching and deposition rate uniformity levels at these very high excitation frequencies and with strong harmonics present has proven to be a difficult feat, for a number of reasons.
For example, as the plasma RF excitation frequency is increased, the wavelength of the RF wave decreases. Thus, RF electromagnetic field spatial variations are more pronounced at these higher frequencies and this adversely affects process uniformity. In addition, another trend in the industry is to process larger wafers, 300 mm diameter wafer technology presently being implemented. Of course, as wafer diameter increases, the wavelength-to-wafer-diameter ratio decreases.
Plasma acts as a nonlinear RF circuit element and thus acts as a source of harmonics of the fundamental excitation frequency. These harmonics, due to their higher frequencies, have an even higher power coupling efficiency to the plasma than the fundamental. Therefore, harmonics, even if present at very low power levels, can significantly affect process uniformity due to their very unfavorable wavelength-to-wafer-diameter ratio.
Since harmonics of the RF fundamental excitation frequency have comparatively short wavelengths, they are far more likely to set up resonances in various places in the process chamber, RF transmission lines, cavities, etc., since their half-wavelengths are comparable to the dimensions of these places.
The situation is further worsened by the use of components made of high permittivity (ε) and/or permeability (μ) materials, or by the presence of RF transmission structural features that have significant series inductance (L) and/or shunt capacitance (C). Both of these effects reduce the wavelength of the propagating electromagnetic wave in a structure, the former by directly changing the wave propagation velocity, the latter by creating a “slow-wave” structure. This wavelength reduction allows harmonics to resonate in places where they normally would not.
It can thus be seen that reduction of the power content of the harmonics of the RF excitation frequency would improve etch or deposition uniformity.